Two retarded groups (Down's Syndrome and etiology unknown) will be compared to matched mental age and chronological age controls. Experiment I will determine the minimum exposure duration necessary for eight consecutive correct identifications of a randomly presented T or A. In experiment I the T or A will be presented to the right, left, and center of the visual field in order to study and compare right and left hemisphere functioning. In Experiment II the test stimulus (T or A) will be presented to each subject at the minimum stimulus exposure duration determined in Experiment I. This procedure provides a control over input and other non-mask variables. In Experiment II the test stimulus will also be presented to the right, left, and center of the visual field. In Experiment II the test stimulus will be followed (Backward Masking) or preceded (Forward Masking) by a non-informational pattern stimulus under five levels of stimulus onset asycroncy (SOA) and a no mask control. The procedure will premit an analysis of a variety of stages of visual information processing in mental retardation including input, brief perceptual memory (e.g., iconic storage), processing speed, and output factors. Results will help determine how and why the mentally retarded are excessively vulnerable to the masking stimulus. Results will also help isolate strengths as well as weaknesses in the information processing system of retarded adults, isolate possible hemispheric weaknesses, and pave the way for the development of improved techniques for the assessment and treatment of persons with special developmental needs.